1. Field of the Invention
The present disclosure relates to rotating machinery, and particularly to rotating machinery suitable for use as a large generator such as, for example, a turbine generator.
2. Description of the Related Art
Generally, a large generator such as a turbine generator is configured from a rotor rotatable together with a rotary shaft and including a rotor iron core and rotor coils, and a stator disposed in an opposed relationship to the rotor with a predetermined gap left therebetween and including a stator iron core and stator coils (armature coils). The stator iron core is configured from stacked steel sheets and has slots formed at predetermined distances in a circumferential direction at the inner circumference side thereof and extending in an axial direction in order to accommodate the stator coils. A tooth exists between each adjacent ones of the slots in the circumferential direction, and the stator coils are accommodated in the slots such that two stator coils are accommodated at the upper and lower sides in a diametrical direction in each slot. The coil accommodated at the inner circumference side is called top coil, and the coil accommodated at the outer circumference side is called bottom coil.
Incidentally, since a generator of a large capacity outputs high current, electromagnetic force and heat generation of the stator coils are high. As a countermeasure for this, a method is taken wherein each stator coil is configured from a plurality of parallel circuits to decrease the current per one coil thereby to moderate electromagnetic force and temperature rise.
However, where the number of parallel circuits is not equal to a divisor of the number of poles, the parallel circuits are deviated in current, resulting in appearance of a deviation in current that circulates between the parallel circuits. The circulating current gives rise to a problem that it increases the loss of the stator coils and raises the temperature of the coils, resulting in a decrease in efficiency or the possibility of damage to coil insulation.
As a method for suppressing such circulating current as described above, a method of changing the combination of coil connections is available. According to this method, the coils are arranged changing the combination of coil connections taking the balance in voltage when the parallel circuits are released into consideration to suppress the unbalance in voltage.
Japanese Patent No. 5060325 (hereinafter referred to as Patent Document 1) can be listed as a document that discloses a technology for suppressing such circulating current as described above to suppress the unbalance in voltage. In Patent Document 1, a connection scheme of stator windings of rotating machinery of the two-pole, three-phase, four-parallel circuit and 84-slot type. According to the connection scheme, the voltage unbalance rate that is the rate of the unbalance in voltage is 0.18%. It is to be noted that the voltage unbalance rate is an index that indicates that, when it is low, the degree of the unbalance in voltage is low.